Wire and plastic conveyor belt systems are widely used in a great variety of industrial fields. A wire mesh or plastic overlay can be used with the rods to provide a support surface of a conveyor belt. The wire or plastic components of the overlay are intermeshed together by connecting rods which extend transversely across the width of the belt. Typically the intermeshed overlay components are connected to one another by the connecting rods. Alternatively, the support surface provided by the rods can be used without an overlay. The ends of the rods are inserted into connective links and the rods and links are welded together. The connective links may serve as tractive links around a sprocket wheel.
In conveyor belts used to move material through processing machinery, such as coating, freezing, cooking, etc., the process cycles have been carefully determined so that the material is conveyed to provide an optimum dwell time in processing. When there is a demand for a higher output of finished product, the most effective way of increasing the output is to enlarge the conveying surface to enable more product or material to be conveyed through the process cycle.
In a system in which the conveyor traverses a spiral path, the conveyor belt is driven by friction on the inside edge of the spiral, and the belt must be sufficiently strong to withstand the resultant driving forces on the links and rods. The tension in the belt is always a design consideration in changing any of the belt parameters such as the size and strength of the links and rods since both of these changes affect the weight of the belt. The heavier the belt, the higher the resultant tension when the belt is driven.
In spiral systems, when a conveyor belt is subjected to tensions above an allowable limit, for instance while turning with an inside edge in a collapsed condition, the forces on the belt components comprise a tangential force on the outer link and a radially directed inward force which pulls the belt against the cylindrical driving surface. If this radially directed force component becomes sufficiently high, the connective rods buckle due to the columnar load placed on them. This is a failure that can occur with any width of belt if the belt is subject to tensions above an allowable limit. For rods above a certain length dimension, it has been observed that the belts will succumb to rod buckling at tensions below the allowable limit. Accordingly, as the width of a belt increases due to the use of longer rods, it takes significantly less tension to buckle the rods.
In a conventional conveyor belt, a current limitation to the size of the conveying surface is the width of the belt due to the column strength of the rods and the potential buckling problem. For example, one of the standards is to use 6-gauge rods which have a uniform diameter along their entire length, and is limited to a maximum length of 38 inches. It has been found that if the rods are made longer than 38 inches, the column strength of the rods poses a weakness to the design and failure by buckling at tensions below the allowable limit can result. This is particularly true in spiral systems which exert a high radial component of force on the rods directed toward the driving surface, that is a force along the length of the rod, and can cause buckling of rods without sufficient column strength.
In conventional conveyor belts, a weld is typically placed at the exterior sides of the link legs to attach the link to the rod. This weld serves two purposes. Firstly, it achieves positive fixation of the link to the outermost portion of the rod. This is important because the connective links at the outer edges of a belt are used for driving interface with the drive sprockets so that any transverse or lateral movement of the connective link along the rod will result in misalignment with the sprockets. This will lead to damage to the sprockets, belt and the system. Secondly, the weld prohibits the connective links from rotational movement that leads to "tenting" of the link on the rod. Tenting refers to the rotational movement of the link relative to the rod when the rods move together causing a link to tilt upward about its rod apertures, and is illustrated schematically in FIG. 15. This movement normally occurs on the inside edge of the belt during its collapsed state of operation in a spiral system, and cause the belt to jam during operation resulting in belt and system damage, as well as downtime.
The weld holding the link to the rod is a main area of perceived failure and real failure for conveyor belts since the weld is subjected to numerous stresses during operation. The biggest contributors to weld failures include normal fatigue caused by belt tensions above the allowable limit including lateral deflection of the legs of the connective links and rod buckling. These stresses on the links cause cracks to form in the welds. Even after a weld fails, however, it continues to function to hold the connective links in a fixed position which allows the belt to operate. It has been observed that the weld deposits on the rod act as barriers that trap the connective link in place allowing for proper sprocket and drive engagement. In addition, while tenting has been observed where welds have failed, in general the ragged edges of the fractured welds continue to prevent rotational movement of the connective link.
A conveyor belt with fractured welds is acceptable and operational as long as the system is running under allowable tension limits, however, the problem of perceived failure of the belt arises whenever some of the welds are cracked. Even though a belt with cracked welds is still able to function effectively under normal operating conditions, users who have been repeatedly warned by belt manufacturers that broken welds are precursors to real failure have perceived the broken welds as a failure in and of themselves. There has been a need to positively position the links and prevent their rotation about the rods, and eliminate this perceived problem with conventional belts that raises user complaints and warranty issues.
Besides showing the first signs of fatigue stress, the conventional welds between the links and rods also pose a time-consuming welding step during manufacture. As can be seen from FIGS. 13-14B, the conventional methods require that the connective links be individually welded to the rods. This step is inefficient and expensive.
Another problem with conventional conveyor belts is that the connective links can break when a belt is subjected to tensions above the allowable limit and the welds have previously fractured. Unlike the situation with fractured welds in which the belt can continue to operate under the allowable tension limits, failure of the connective links is the final failure mode of the belt. If the belt is operated at excessive tension above the allowable limit after the welds have fractured, the links will fail.
Since a belt is driven by friction to friction, the connective links are prone to fatigue stresses due to the resultant tension on them. This is because when a belt is driven, the driving friction on the belt tends to draw the belt components together laterally. The periodic starting and stopping of the belt which is inherent to any operation causes the legs of a connective link to deflect laterally with respect to the rod, and to move rotationally with respect to the rod. Additionally the changes in belt tension through the system tend to cause the legs of the connective links to repeatedly defect laterally. The lateral deflection refers to the tendency of the link legs to move inward, toward one another when the belt is driven and to spread outward when the driving friction on the belt is released. The continual movement causes fatigue stresses in the connective link and ultimately to catastrophic link failure. When this occurs, the belt may physically separate resulting in damage to the system and large portions of the belt as well as downtime.
Both the lateral deflection and rotational movement of the link about the rod, i.e. tenting, during operation can result in fatigue failure of the link.
Therefore, there exists a need for a conveyor belt that can provide a larger surface for moving more products or articles through process cycles with structural features that reduce weld failure, decrease deflection and tenting of the links in order to resist fatigue failures, and increase the column strength of the rods to resist buckling. Enlarging the size of the surface is the preferred way to increasing output volume because the process cycles do not have to be altered from their predetermined settings. There also exists a need for streamlining the manufacture of conveyor belts by eliminating the current methods of attaching the connective links to the rods.